What happens if I miss the Self Assessment deadline?

You get an automatic £100 penalty the moment you're even one day late filing, regardless of whether you owe any tax at all, and further daily penalties of £10 accrue after three months, with larger penalties on top after six and twelve months.

Separately, if you owe tax and don't pay it by 31 January, interest starts accruing immediately and a 5% late-payment penalty applies at 30 days, 6 months and 12 months if the balance is still outstanding. These are two entirely separate penalty regimes — filing late and paying late — so it's possible to be hit by both even on the same return. If you know you're going to miss the deadline, filing as soon as possible afterwards still limits the damage since the penalties are time-based, and if you have a genuine reasonable excuse HMRC can sometimes waive the filing penalty, though this is assessed case by case and isn't guaranteed. If you're behind on a return right now, getting it filed — even late — stops the position getting worse.